Factors influencing the importance of Gammarus spp. (Crustacea: Amphipoda) in riverine salmonid diets

Abstract

Spatial (river site) and temporal (seasonal/diel) variations in the relative abundance of native and introduced Gammarus spp. (amphipods) compared to all other macroinvertebrates in the benthos, drift and gut contents of resident salmonids, were investigated in eight Northern Ireland rivers. Despite marked seasonal and diel changes in salmonid diets, Gammarus was a minor prey item, representing <5 % of all prey items in stomachs at 7 sites dominated by the native species Gammarus duebeni celticus. In these sites, larval Baetis spp. (Ephemeroptera) and Chironomidae (Diptera) represented the main components of fish diets. In contrast, in a site where the introduced species G. pulex dominated, it formed between 75 and 98 % of 1+ and 2+ trout stomach samples. The fish of this site also had consistently lower dietary diversities than fish at all other sites, reflecting a lower diversity in the macroinvertebrate community. Salmonids rarely showed distinct preferences for Gammarus, except in the site dominated by G. pulex. Indeed, there was an avoidance of Gammarus by fish in many rivers. Therefore, the relative abundance of Gammarus spp. in salmonid diets may ultimately depend on its availability' in the environment and the range of alternative prey species present. This study also highlights the ambiguous nature of the outcomes of attempting to manipulate fish diets by introducing new prey species.